This invention relates to selective light absorbing surfaces or gratings.
U.S. Pat. Nos. 6,281,514, 6,495,843, and 6,531,703 disclose methods for promoting the passage of electrons at or through a potential barrier comprising providing a potential barrier having a geometrical shape for causing quantum interference of electron de Broglie wave. Also provided is an electron-emitting surface having a series of indents, the depth of which is chosen so that the probability wave of the electron reflected from the bottom of the indent interferes destructively with the probability wave of the electron reflected from the surface. This results in the increase of tunneling through the potential barrier. A further embodiment provides a method for making an electron-emitting surface having a series of indents.
U.S. Pat. Nos. 6,680,214 and 7,166,786 disclose methods for the induction of a suitable band gap and electron emissive properties into a substance, in which the substrate is provided with a surface structure corresponding to the interference of de Broglie waves.
WO99/064642 discloses a method for fabricating nanostructures directly in a material film, preferably a metal film, deposited on a substrate.
WO04/040617 discloses a method that blocks movement of low energy electrons through a thermoelectric material. This is achieved using a filter that is more transparent to high-energy electrons than to low energy ones. The geometry of the filter is such that it becomes transparent for electrons having certain de Broglie wavelength. If the geometry of the filter is such that its transparency wavelength matches the wavelength of high-energy electrons it will be transparent for high-energy electrons whilst blocking low energy electrons.
Semiconductors are characterized by the presence of an energy gap between the occupied valence band and largely empty conduction band. This energy gap is a forbidden zone within which electrons cannot exist. Incident radiation with photon energy greater than the energy gap is absorbed by electrons in the valence band. These electrons are excited to the conduction band, leaving behind a positively charged ion, known as a hole, in the valence band.
Doping of a semiconductor allows the existence of energy levels, and therefore electrons, within the forbidden zone. The exact location of these energy levels depends on the dopant and its concentration. Doping with electron donors (n-type doping) produces energy levels close to the conduction band whereas doping with hole donors (p-type doping) produces energy levels closer to the valence band. These energy levels, when occupied, have a relatively long lifetime and electrons can therefore accumulate in these energy levels.
The semiconductor properties described above can be better understood with reference to FIG. 1. Shown is a doped semiconductor within which we have bottom of conduction band 12, energy level 13 created in the previously forbidden zone and top of valence band 11. Incoming radiation with energy greater than or equal to the energy gap between levels 11 and 12 excites electrons from valence band to conduction band. These electrons then descend via thermal or irradiative losses to energy level 13 where they accumulate due to the relative stability of energy level 13. When electrons drop from energy level 13 back to top of valence band 11, they recombine with holes and a photon with energy equal to the energy gap between levels 13 and 11 is emitted.
Clearly, the frequency of the emitted radiation is lower than the frequency of the incident radiation.
Solar cell technology is based on the semiconductor p-n junction. Light is absorbed in the semiconductor causing transition of electrons from the valence band of the semiconductor to the conduction band, as shown in FIG. 2. The relationship between the energy of the photon, hν, and the gap width in the energy spectrum of the semiconductor, Eg, defines the mechanism of photon absorption. In the case where hν<Eg, the photon is not absorbed inside the semiconductor and the semiconductor layer appears to be transparent for light of that wavelength. In the case where hν=Eg, the photon causes transition of electrons from the valence band to the conduction band. Those electrons are collected by another electrode and their energy is converted into electric energy (not shown in FIG. 2). In the case when hν>Eg, the photon will excite the electron from the valence band to conduction band with excess of kinetic energy Eh. This electron will fall back to the bottom of conduction band releasing excess energy as heat in light collecting electrode. In the last case some of the energy of the incoming photon is converted to heat instead of electricity reducing the efficiency of the device.
The semiconductor layer is thus only converting photon energy efficiently in the narrow band of photon energies of incoming light (when hν˜Eg). To solve this problem multistage energy conversion has been used, as shown in FIG. 3, in which the device comprises many layers placed in series normal to the incoming light. Each layer comprises a p-n junction and is electrically connected in series. The first layer has the widest energy gap in the spectrum. It absorbs photons having energy hν1 and is transparent for photons having lower energies hν2, hν2<hν1. The next layer absorbs photons having energy hν2 and is transparent for photons having energy hν3<hν2 and so on. It is thus clear that each layer and the device as a whole will be most efficient if each layer absorbs only its characteristic frequency ν1=Eg1/h.